Change Gears for Threading

The series of gears that drive the lead screw are called change gears because you change them to turn different thread pitches.

Changing these gears varies the speed that the lead screw turns in relation to the speed that the spindle turns. This allows you to cut threads with different numbers of threads per inch.

This calculator determines the correct change gears for any thread pitch. In some cases, the combination of gears given will produce an approximation of the pitch you need. In these cases the error from the desired pitch is shown. Note that in most cases these variations are very small and the resulting thread will meet industry standards.

Change Gear Calculator

Make the choices to define your lathe below before calculating a thread pitch.

Desired Thread Pitch		TPI mm		Enter the desired pitch in TPI (Threads Per Inch) or millimeters (mm from crest to crest). Be sure to click the correct units option (TPI or mm).
				Lathe settings are at the bottom of this page.

Change Gear Results

Other Lathe - 1.5 mm thread pitch
Lathe settings are at the bottom of this page.

No valid result found

Instructions

There are 4 positions for the change gears, commonly called A, B, C, and D.

A	This is the top change gear position. When you received your lathe it had a 20-tooth metal gear in this position.
B	Gear positions B and C are on the same shaft, between positions A and D. Position B is the inside gear on this shaft. When you received your lathe it had an 80-tooth plastic gear in this position.
C	Gear positions B and C are on the same shaft; between positions A and D. Position C is the outside gear on this shaft. When you received your lathe it had a 20-tooth metal gear in this position.
D	Position D is the end of the lead screw. When you received your lathe it had an 80-tooth plastic gear in this position.

Change gear positions.

The change gears are commonly tight on the shaft when new. You might need to use a screwdriver behind them to pry them off.
Gear positions B and C are on a hollow shaft that comes off easily when the retaining socket head cap screw is removed. Then you can use an arbor press to remove the gears from the hollow shaft. Be careful that you do not lose the key.

Gear position D has a spacer behind the gear. When you only use three gears, put the spacer outside the gear so the gear will align with the gear in position B.

In the change gear results, many of the combinations have "Any gear" in columns B and C. This means that you can use a gear with any number of teeth in position B. It is an idler and does not affect the overall gear ratio. Use a gear that makes it easy to properly engage the gears. For these combinations, you can use any gear for position C; this gear acts only as a spacer and does not engage the other gears.

Lathe

Choices

Notes

7x Mini Lathe

Lead screw pitch:
16 TPI 1.5 mm
Mini lathes sold in the US have 16 TPI lead screws (known as "inch" or "imperial"). Mini lathes sold in most other parts of the world have 1.5 mm pitch (metric) lead screws.

Additional gears:
List the number of teeth on any non-standard gears you have (if any), separated by commas (,). The normal set of change gears includes the following gears: 20, 20, 35, 40, 40, 45, 50, 55, 57, 60, 65, 80, and 80 teeth.

Choose 7x Mini Lathe for most 7x10, 7x12, 7x14, and 7x16 mini lathes.

HiTorque Mini Lathe
(with 21 Tooth Change Gear)

Choose HiTorque Mini Lathe for LittleMachineShop.com HiTorque mini lathes and other mini lathes when a 21 tooth gear is available.
A 21-tooth gear can provide closer approximations of metric gears for some desired pitches.

HiTorque Bench Lathe

Use this choice for the LittleMachineShop.com HiTorque 8.5x16 and 8.5x20 bench lathes.

Other lathes
(enter all values)

Change gear set:

Lead screw pitch:
TPI mm

A-B min:
A-B max:
C-D min:
C-D max:

This calculator will work for most lathes that use four change gears. Enter all gears in the change gear set, including duplicates and the gears that are mounted on the lathe for regular turning.
A-B min is the total number of teeth on the two smallest gears you can mount in the A & B positions. A-B max is total number of teeth on the two largest gears you can mount in the A & B positions. Likewise C-D min and C-D max. If you don't enter anything we will take a guess, but it might produce results that are physically impossible to mount.
Try a couple of thread pitches for which you know the answer to ensure you are getting correct results.

Have you seen?

Electronic Digital Indicator, 0.5"